Electro-wetting on dielectric (EWOD) is a well known technique for manipulating droplets of fluid by application of an electric field. It is thus a candidate technology for digital microfluidics for lab-on-a-chip technology. An introduction to the basic principles of the technology can be found in “Digital microfluidics: is a true lab-on-a-chip possible?”, R. B. Fair, Microfluid Nanofluid (2007) 3:245-281).
FIG. 1 shows a part of a conventional EWOD device in cross section. The device includes a lower substrate 72, the uppermost layer of which is formed from a conductive material which is patterned so that a plurality of electrodes 38 (e.g., 38A and 38B in FIG. 1) are realized. The electrode of a given array element may be termed the element electrode 38. The liquid droplet 4, consisting of a polar material (which is commonly also aqueous and/or ionic), and is constrained in a plane between the lower substrate 72 and a top substrate 36. A suitable gap between the two substrates may be realized by means of a spacer 32, and a non-polar fluid 34 (e.g. oil) may be used to occupy the volume not occupied by the liquid droplet 4. An insulator layer 20 disposed upon the lower substrate 72 separates the conductive element electrodes 38A, 38B from a first hydrophobic coating 16 upon which the liquid droplet 4 sits with a contact angle 6 represented by θ. The hydrophobic coating is formed from a hydrophobic material (commonly, but not necessarily, a fluoropolymer).
On the top substrate 36 is a second hydrophobic coating 26 with which the liquid droplet 4 may come into contact. Interposed between the top substrate 36 and the second hydrophobic coating 26 is a reference electrode 28.
The contact angle θ 6 is defined as shown in FIG. 1, and is determined by the balancing of the surface tension components between the solid-liquid (γSL), liquid-gas (γLG) and non-ionic fluid (γSG) interfaces, and in the case where no voltages are applied satisfies Young's law, the equation being given by:
                              cos          ⁢                                          ⁢          θ                =                                            γ              SG                        -                          γ              SL                                            γ            LG                                              (                  equation          ⁢                                          ⁢          1                )            In certain cases, the relative surface tensions of the materials involved (i.e. the values of γSL, γLG and γSG) may be such that the right hand side of equation (1) is less than −1. This may commonly occur in the case in which the non-polar fluid 34 is oil. Under these conditions, the liquid droplet 4 may lose contact with the hydrophobic coatings 16 and 26, and a thin layer of the non-polar fluid 34 (oil) may be formed between the liquid droplet 4 and the hydrophobic coatings 16 and 26.
In operation, voltages termed the EW drive voltages, (e.g. VT, V0 and V00 in FIG. 1) may be externally applied to different electrodes (e.g. element electrodes 38, 38A and 38B, respectively). The resulting electrical forces that are set up effectively control the hydrophobicity of the hydrophobic coating 16. By arranging for different EW drive voltages (e.g. V0 and V00) to be applied to different element electrodes (e.g. 38A and 38B), the liquid droplet 4 may be moved in the lateral plane between the two substrates 72 and 36.
In the following description, it will be assumed that an element of an EWOD device, such as the device of FIG. 1, may receive “digital” data so that the element is required to be put in either an “actuated” state (in which the voltage applied across the element is sufficient for a liquid droplet in the element (if one is present in the element) to experience a significant electro-wetting force) or a “non-actuated” state (in which the voltage applied across the element is not sufficient for a liquid droplet in the element (if one is present in the element) to experience a significant electro-wetting force). An element of an EWOD device may be put into the actuated state by applying a voltage difference across the EWOD element having a magnitude that is equal to, or greater than, a threshold voltage VEW, whereas if the voltage difference across the EWOD element has a magnitude that is less, or much less than the threshold voltage VEW the element is in its non-actuated state. The threshold voltage VEW is often referred to as an “actuation voltage”, and this term is used below. In practice the threshold voltage may typically be determined as the minimum voltage required to effect droplet operations, for example the moving or splitting of droplets. In many cases there is one threshold voltage for droplets to move and a second (higher) threshold voltage for droplets to split, and in such cases the “actuation voltage” is preferably set above the threshold required for droplets to split. In practice the non-actuated state may typically be zero volts.
Typically EWOD systems may be considered to be digital, in that the EWOD elements are programmed either to an actuated state or a non-actuated state. However, the actuation due to electro-wetting is essentially analogue in nature, so the actuation force can be tuned by varying the voltage (up to a certain maximum voltage at which the actuation force saturates). Some performance parameters also depend in an analogue manner on voltage—for example the maximum speed of movement of a droplet is approximately proportional to the applied voltage. It should therefore be understood that an EWOD device may alternatively be operated by supplying analogue input data rather than digital data.
FIG. 2 shows a circuit representation of the electrical load presented between the element electrode 38 and the reference electrode 28. The liquid droplet 4 can be modelled as resistor and capacitor in parallel, the hydrophobic coatings 16 and 26 as capacitors and the insulator 16 as a capacitor. For the purposes of driving and sensing the electrical load functions effectively as a capacitor whose value depends on whether a liquid droplet 4 is present or not a given element electrode 38.
U.S. Pat. No. 6,565,727 (Shenderov, issued May 20, 2003) discloses a passive matrix EWOD device for moving droplets through an array.
U.S. Pat. No. 6,911,132 (Pamula et al., issued Jun. 28, 2005) discloses a two dimensional EWOD array to control the position and movement of droplets in two dimensions.
U.S. Pat. No. 6,565,727 further discloses methods for other droplet operations including the splitting and merging of droplets, and the mixing together of droplets of different materials.
U.S. Pat. No. 7,163,612 (Sterling et al., issued Jan. 16, 2007) describes how TFT based thin film electronics may be used to control the addressing of voltage pulses to an EWOD array by using circuit arrangements very similar to those employed in AM display technologies.
The approach of U.S. Pat. No. 7,163,612 may be termed “Active Matrix Electro-wetting on Dielectric” (AM-EWOD). There are several advantages in using TFT based thin film electronics to control an EWOD array, namely:                Electronic driver circuits can be integrated onto the lower substrate 72        TFT-based thin film electronics are well suited to the AM-EWOD application. They are cheap to produce so that relatively large substrate areas can be produced at relatively low cost        TFTs fabricated in standard processes can be designed to operate at much higher voltages than transistors fabricated in standard CMOS processes. This is significant since many EWOD technologies require EWOD actuation voltages in excess of 20V to be applied.        
EP2404675 (Hadwen et al., published Jan. 11, 2012) describes array element circuits for an AM-EWOD device. Various methods are known for programming and applying an EWOD actuation voltage to the EWOD element electrode 38. The voltage write function described includes a memory element of standard means, for example, based on Dynamic RAM (DRAM) or Static RAM (SRAM) and input lines for programming the array element.
Whilst EWOD (and AM-EWOD) devices can be operated with either DC or AC actuation voltages, in practice there are many reasons for preferring an AC method of driving, as reviewed in the previously cited reference R. B. Fair, Microfluid Nanofluid (2007) 3:245-281. It may be noted that droplets can be actuated and manipulated for a wide range of AC driving frequencies ranging typically from a few hertz to several kHz.
One possible method for implementing an AC driving method in an AM-EWOD device is to apply a ground potential to the reference electrode 28. To program a given element electrode in the array to a non-actuated state, the element electrode is programmed to a ground potential. To program a given array element electrode 38 to an actuated state, the element electrode potential 38 is programmed to alternate between a potential of VEW and −VEW. However this method of driving has the significant disadvantage that the maximum voltage that must be switched by the transistors in the circuit in order to drive the element electrode 38 is required to be 2VEW.
U.S. Pat. No. 8,173,000 (Hadwen et al., issued May 8, 2012) describes an AM-EWOD device with array element circuit and method for writing an AC actuation voltage to the electrode. The AC drive scheme described by this patent utilizes the application of AC signals to both the element electrode 38 and to the reference electrode 28 of the device. Therefore, the device is capable of generating a voltage difference between the electrodes that varies between +VEW and −VEW, whilst the transistors in the array element circuit are only ever required to operate with a rail-to-rail voltage of VEW.
U.S. Pat. No. 8,653,832 describes how an impedance (capacitance) sensing function can be incorporated into the array element. The impedance sensor may be used for determining the presence and size of liquid droplets present at each electrode in the array.
U.S. Pat. No. 8,221,605 describes a coplanar electrode arrangement wherein the reference electrode is omitted from the top substrate and is replaced by an in-plane reference electrode which is disposed upon the lower substrate along with the drive electrodes. U.S. Pat. No. 8,221,605 discloses how the reference electrode may be comprise a two dimensional grid of conducting lines electrically and physically distinct from the drive electrodes.
U.S. Pat. No. 8,419,273 describes an AM-EWOD device with a temperature sensor integrated into each array element.
Ui-Chong Yi et al. report, in “Characterization of electrowetting actuation on addressable single-side coplanar electrodes”, Journal of Micromechanics and Microengineering, Vol 16, p 2053-2059 (2006), electrowetting-on-dielectric actuations on coplanar electrodes with an electrode-free cover plate or no cover plate. By arranging driving and reference electrodes on one plate, such a configuration can accommodate more sensing mechanisms from above and thus allows increased flexibility for system development. Various coplanar electrodes are tested for contact angle changes with a focus on the effect of the percentage gap between electrodes.
US 2013/0271153 describes an AM-EWOD device in which each array element circuit includes a top substrate electrode and a drive electrode between which the one or more droplets may be positioned, with an insulator layer being interposed between the droplet(s) and the drive electrode, and write circuitry. At least some of the array element circuits include measure circuitry configured to detect a pinhole defect in the insulator layer.
WO 2008/101194 describes a method of determining the presence, partial presence or absence of a droplet at an electrode on a droplet actuator by: (a) providing a droplet actuator comprising: (i) a substrate comprising electrodes arranged on the substrate for conducting droplet operations on a surface of the substrate; and (ii) a capacitance detection circuit for detecting capacitance at the droplet operations surface at one or more of the electrodes; (b) detecting capacitance at the droplet operations surface at one or more of the electrodes; and (c) determining from the capacitance the presence, partial presence or absence of a droplet at the droplet operations surface at the electrode.